Varactors are voltage tunable capacitors in which the capacitance is dependent on a voltage applied thereto. Although not limited in this respect, this property has applications in electrically tuning radio frequency (RF) circuits, such as filters, phase shifters, and so on. The most commonly used varactor is semiconductor diode varactor, which has the advantages of high tunability and low tuning voltage, but suffers low Q, low power handling capability, and limited capacitance range. A new type of varactor is a ferroelectric varactor in which the capacitance is tuned by varying the dielectric constant of a ferroelectric material by changing the bias voltage. Ferroelectric varactors have high Q, high power handling capacity, and high capacitance range.
One ferroelectric varactor is disclosed in U.S. Pat. No. 5,640,042 entitled “Thin Film Ferroelectric Varactor” by Thomas E. Koscica et al. That patent discloses a planar ferroelectric varactor, which includes a carrier substrate layer, a high temperature superconducting metallic layer deposited on the substrate, a lattice matching, a thin film ferroelectric layer deposited on the metallic layer, and a plurality of metallic conductors disposed on the ferroelectric layer and in contact with radio frequency (RF) transmission lines in tuning devices. Another tunable capacitor using a ferroelectric element in combination with a superconducting element is disclosed in U.S. Pat. No. 5,721,194. Tunable varactors that utilize a ferroelectric layer, and various devices that include such varactors are also disclosed in U.S. Pat. No. 6,531,936, entitled “Voltage Tunable Varactors And Tunable Devices Including Such Varactors,” filed Oct. 15, 1999, and assigned to the same assignee as the present invention.
A major concern in tunable capacitors including voltage tunable dielectric capacitors is the elimination of losses due to the electrostrictive effect and acoustic resonances. These resonances typically manifest as a series of bumps and ripples on the Q vs. Frequency characteristic of a tunable capacitor causing the Q-factor to dip as low as 10 to 20.
A model of dielectric losses due to the electrostrictive effect and acoustic resonances has been proposed with Electrostrictive resonances in Ba0.7Sr0.3.TiO3 thin films at microwave frequencies which uses very thin BST layers, which would ensure a sufficiently high fundamental frequency of the acoustic resonance—i.e. beyond the frequency range of the application.
However, the aforementioned thin BST layers are impractical due to manufacturability and linearity considerations. Hence, what is needed is an apparatus and method capable of a high fundamental acoustic resonance frequency and a wide resonance-free frequency range using films that are readily manufacturable.